Couplers are used in high-frequency devices to add or remove power from one conductor to another conductor. A variety of couplers have been developed, including branch-line couplers, Bethe couplers, and Lange couplers. Couplers have been developed based on a variety of transmission line structures, including waveguide transmission structures, coaxial transmission structures, and strip-line transmission structures. Generally, a portion of a first signal in one conductor is coupled to the other conductor to produce a second signal that propagates opposite to the direction of propagation in the first conductor. Ideally, any signal propagating in the second conductor in the same direction as the first signal cancels itself out in the forward direction but not in the reverse direction. In reality, some energy will propagate in the second conductor in the same direction as the first. The directivity of a coupler is a figure of merit that indicates the energy in the second conductor propagating in the desired direction (i.e. opposite the direction of propagation in the first conductor) relative to the energy propagating in the opposite direction.
Many couplers are based on a planar geometry that has two conductors defined in close proximity on a non-conductive substrate, such as a thin-film substrate, thick-film substrate, printed circuit board (“PCB”) substrate, or semiconductor wafer. Unfortunately, electromagnetic energy from the conductors couples into the substrate material, resulting in loss. Similarly, coupling energy into the substrate typically degrades directivity of the coupler.
Couplers have been designed that suspend the conductors of the coupler in air, or suspend one of the conductors in air and define the second conductor on a substrate. Dielectric beads, pins, or pegs are used to support conductors of a coupler in a package (housing) of a microcircuit; however, such couplers are specific to a particular package because the supports are placed in the package with a high degress of precision or adjustability. This increases manufacturing costs because new pegs are designed for each new package configuration. Furthermore, the material of the support material (pegs) preferentially retards the propagation of the even transmission mode relative to the odd transmission mode, which degrades performance of the coupler. It is also difficult to reduce the size of such designs to produce couplers suitable for very high frequency operation.
Thus, couplers that avoid the problems of the prior art are desirable.